In the production of semiconductor wafers, many coating and etching processes are commonly performed in a vacuum environment with the use of a plasma. Various methods of producing plasmas result in the production of ions and electrons at various energy levels, with varying degrees of efficiency and with varying degrees of spatial uniformity within a plasma processing chamber.
In processes such as sputter coating of wafers, plasmas are directed onto a target of sputtering material to eject material particles of the sputtered target material from the target for deposition onto a wafer substrate. In sputter etching, a plasma is directed onto the wafer substrate where the ions eject surface material from the wafer. In etching processes, plasmas may be used to selectively remove portions of hard layers of material from the substrate with high energy ion bombardment from plasmas, or may be used for light or "soft" etching to remove, without damaging the wafer surface, only a thin surface layer from the wafer to clean or condition the wafer surface for subsequent processing. Soft etching is better preformed with a lower energy plasma. Other reactive processes such as chemical vapor deposition processes (CVD) may employ plasmas in assisting the chemical process.
In sputter coating or hard sputter etching processes, the plasmas are generally produced locally with electrons emitted from a cathode by negatively charging the target of coating material, in sputter coating processes, or the substrate to be etched. Plasmas used for sputter coating processes are typically of relatively high energy, usually in the range of from 400 to 500 electron volts, while those used for hard sputter etching are typically in the range of from 200 to 300 electron volts. These plasmas tend to be of moderately low densities, usually of about five percent ionization.
In semiconductor wafer processing with plasmas, the distribution of the plasma over the surface of a wafer or target must conform to some desired profile. In soft etching, for example, it is usually preferred that a plasma be directed uniformly over the entire surface of the wafer. In addition, since in soft etching processes it is usually undesirable to remove substantial amounts of material from, or otherwise damage, the substrate, it is desirable that the plasma impinge upon the substrate with low energy, usually of less than 100 electron volts. As a result, the heating effects of the plasma striking the substrate are relatively low, allowing the use of plasmas of higher densities to produce a higher etching rate.
The production of plasmas by some processes, such as by high frequency RF excitation of the gas in the chamber, often results in a plasma in which the ions are at several hundred electron volts. Even when magnetron enhanced, the plasma production efficiency is usually not more than from five to ten percent ionization. On the other hand, production of plasmas with procedures such as electron cyclotron resonance may produce a plasma of from 15 to 35 electron volts and of from 25% to 30% ionization efficiency.
Commonly, ECR generation of plasmas is accomplished at a location remote from the surfaces being treated with the plasma, from which it is then transferred to the processing location, usually in a chamber or region adjacent the region of plasma generation. ECR generated plasmas are frequently produced with microwave energy, often at the available FCC assigned frequency of 2.54 GHz. The microwave energy is introduced into a vacuum cavity that is often cylindrical in shape, and to which a magnetic field is applied by permanent magnets or coils positioned outside of a cylindrical chamber wall. At the 2.54 GHz frequency, resonance of electrons can be achieved in a magnetic field of about 875 gauss.
In many ECR plasma generators, the magnetic fields are generated by electromagnet coils or solenoids wound around the outside of the cylindrical cavity wall to produce in a cavity a magnetic field with lines of force extending axially within the cavity, circling out of the ends of the cavity and around the windings on the outside of the cavity. Within the cavity, magnetic fields so produced are generally symmetrical about the center or axis of the cavity and produce a similarly symmetrical plasma around the axis of the cavity.
Magnetic fields of other shapes are more easily produced with permanent magnets positioned around the outside of the cavity. Examples of such fields are cusp fields, which have field lines extending through the side walls of the cavity. These cusp fields tend to have symmetries about one or more planes that pass through or contain the axis of the cavity and intersect the chamber wall.
With magnets having radially oriented polar axes positioned around a cylindrical cavity, each with the same polar orientation, a single-cusp field is produced. In this single-cusp field, the lines of force diverge from the cylinder wall proximate the centers of the magnet poles along a circle around the cylindrical chamber wall, forming two field portions that are symmetrical at a plane that contains this circle and bisects the axis of the cylinder. From this plane, the lines of magnetic force of the field extend axially out of the ends of the chamber and return to the opposite poles of the magnets outside of the chamber, with the stronger resonance supporting lines of force nearer the plane and the axis of the cavity, producing a pair of adjacent resonance regions on opposite sides of the plane and in which plasmas are generated. The shape of the plasma is generally defined by the cusp formed between the diverging field lines.
With similarly positioned magnets but alternately oriented in polarity, a multicusp field is produced in which the lines of force emerge from alternate and similarly oriented ones of the magnets along longitudinal lines on the cylinder wall proximate the magnet poles, loop in radial plane toward the adjacent and oppositely oriented magnets, and reenter the wall along lines longitudinal lines proximate the poles of these adjacent magnets. Such fields tend to be weak along the axis of the cavity and produce distinct plasmas, symmetric about the planes and concentrated in wedge shaped regions between the planes.
While the permanent magnets configured to produce these fields are simple and compact, the fields that they produce and the distributions of the plasmas generated in them are complex with many non-uniformities.
In magnet configurations suitable for use in plasma generators, the regions of electron cyclotron resonance, and consequently the areas at which the plasma is most abundantly produced, are not distributed within the resonance cavity with perfect uniformity. This is primarily due to practical limitations in the precision to which the magnets are made and mounted and to which the cavity is constructed, and to non-uniformities in the distribution and flow of gas within the chamber. As a result, the prior art has employed various methods to deal with the non-uniform plasma distribution to produce a useful plasma distribution at a space within a processing chamber where the process is to take place. Prior art efforts have been directed toward controlling the flow of the plasma, and its distribution, in the space between the plasma generation region and the processing location. These prior art efforts have resulted in approaches that are undesirable in many ways, being complex, expensive, or less than satisfactory in performance. In cluster tool configurations particularly, where each of the processing chambers is modular, complexity and excess size are disadvantageous.
Accordingly, their is a need for a method and apparatus for the more efficient and uniform production of a plasma for wafer processing, and particularly for a compact and simple plasma generator for such applications.